This invention relates generally to amplifier circuits and more particularly to bipolar transistor amplifier circuits adapted to operate with low supply voltages.
As is known in the art, it is sometimes desirable to provide bipolar transistor amplifier circuits which are able to operate with relatively low voltage supplies, typically 1.5 volt supplies. Further, since this 1.5 volts is the nominal voltage of the supply, as the supply, typically a dry cell battery, ages the voltage decreases and hence it is generally required to design the circuit so that it operates properly with as little as 1.0 to 1.1 volts. One type of amplifier circuit includes a pair of transistors arranged as a complementary emitter follower output stage; one of such transistors being an NPN transistor and the other a PNP transistor. The emitter electrodes are connected together at an output terminal and the collector electrodes are connected to opposite polarities of a voltage supply. An input signal is coupled to the base electrode of a third transistor, the emitter and collector electrodes thereof being connected between the collector and base electrodes of the PNP transistor. A diode connected PNP transistor provides a current source between the collector and base of the NPN transistor. With such arrangement, the supply voltage is normally limited to 2(V.sub.CE +V.sub.BE) or 1.8 volts. In another type of amplifier circuit, a PNP and an NPN transistor are provided with the collector electrodes connected together at a common output terminal. The emitter of the PNP transistor is connected to the positive polarity of the voltage supply and the emitter electrode of the NPN transistor is connected to the negative potential of the supply. A pair of resistors is connected serially between the base electrodes of the transistors and an input signal is fed to the junction between the pair of resistors. With such arrangement, since the bias current through the resistors is a function of the supply voltage, in order to provide a bias current, the supply voltage is limited to a minimum level of 2V.sub.BE or about 1.1 volts.